The present invention relates to an apparatus and method for improving retention and reducing decay of data recorded on perpendicular magnetic media.
Data storage devices, such as hard disks, are used to store data, instruction sets and computer programs. A typical disk drive comprises a number of disks each having a magnetic media surface coupled with a recording head. The magnetic media surface comprises magnetic material which typically stores data as one of two distinct magnetization statesxe2x80x94corresponding to 0 and 1 of the data signal. The recording head recognizes recorded bits of information by detecting pulse peaks within certain timing windows. However, systems inadvertently shift pulse patterns, in time, with respect to the timing of the windows and this results in errors. It follows that when linear density is increased, the time windows in which the pulse peak must be detected are narrower and the systems become more sensitive to noise and there are resulting errors.
Data recording systems become even more prone to error when the data storage capacity of the magnetic media is increased. The data storage capacity is increased by increasing the areal density of data which is a function of the number of tracks that can be packed onto the magnetic media surface and the number of bits of data that can be stored per linear inch. Higher areal densities can be facilitated by reducing the size or volume of the grains (individual crystals) on the magnetic media surface. However, as the volume of a grain becomes smaller, the grain becomes more susceptible to spontaneous switching of its state of magnetization at room temperature or with the application of even a small amount of external energy, such as a rise in temperature or the presence of demagnetizing fields. In general, the energy barrier to spontaneous switching of a magnetized grain is equal to the product of its magnetic anisotropy (K) and the volume (V) of the grain. Random thermal energy causes the magnetization direction to become random at a rate, R=w exp(-Emagnet/kT), where w is the xe2x80x9cattempt frequencyxe2x80x9d (about 109/sec), k is Boltzmann""s constant, and T is the absolute temperature. In newly developed magnetic media, the volume of each individual grain is reaching a sufficiently small size that the state of magnetization of the grain can spontaneously switch due to thermoagitation at the operating temperature of the storage device. Such spontaneous switching of magnetization states is undesirable because it gives rise to a decay of the quality of the data over time and the ultimate loss of recorded data.
The spontaneous switching of magnetization states is reduced by recording on perpendicularly oriented data storage media which has perpendicular uniaxial anisotropy which means that the grains are oriented in a substantially perpendicular direction to the plane of the media. In perpendicular recording, write heads record data by having the perpendicularly oriented media pass between a write pole and its mirror image in a magnetically soft underlayer. The perpendicularly oriented grains are taller and have a larger grain volume for a given diameter and consequently are more resistant to spontaneous switching of domains. In addition, the writeable coercitivity of perpendicular media with a soft underlayer can be 2 to 3 times that of longitudinal media. Thus, the volume of a thermally stable grain can be reduced, thereby reducing the area of a bit to achieve higher density. Thus perpendicular recording methods provide a significantly higher areal density than conventional longitudinal recording methods because they provide a gain in thermally stable data density which is two to four times that provided by conventional longitudinal recording methods. Perpendicular recording media utilizing, for example, cobalt-chrome alloys can replace conventional CoCTa longitudinal media (presently used) and achieve densities greater than 30 MB/cm2. In these alloys, the crystal structure is hexagonal. They have an excellent ability to produce small, independent magnets, which have a moderately high anisotropy energy K of about 1xc3x97106 ergs/cm3.
However, in perpendicular recording media, it is believed that a large demagnetization field (Hd) occurs in the long run length regions of the data signal recorded on the media. By xe2x80x9clong run lengthsxe2x80x9d it is meant a region of the recorded data signal that has a constant value of magnetization field strength, such as a region having predominantly, or substantially only, bits corresponding to the xe2x80x9c0xe2x80x9d state. In these regions, the state of magnetization of each grain within the region, experiences a self-demagnetization force because the grain is magnetized perpendicular to the plane of the media and has an inherent demagnetization field that is trying to switch back its magnetization state, the strength of the demagnetization field being equal to the magnetization strength of the grain. Thus, for example, a portion of the media magnetized with 5000 Gauss has a demagnetization field trying to switch it back which is also 5000 Oer. The demagnetization field tends to reverse or switch back the magnetization state of the grain and consequently the presence of the field reduces the energy barrier required for demagnetization of the grain. Thus, the long run length regions are particularly sensitive to any increase in thermal energy which would further lower the energy barrier required to promote demagnetization of the grain, and consequently cause thermal decays in the data signal.
Accordingly, it is desirable to have an apparatus and method of storing data on data storage media that maximizes its data storage capacity. It is further desirable to reduce decay of the data signals stored on the data storage media, especially in the long run length regions of the data. Therefore, it is desirable to reduce the demagnetizing fields in these regions, especially on perpendicular media. In particular, it is desirable to have a disk drive with increased capacity, reduced error rates, and high level of retention of signals.
An apparatus and method according to the principles of the present invention maximizes the data storage capacity of magnetic media while reducing decay of the data signals stored on the media, especially in the long run length regions of the data. In one aspect, the present invention comprises a data storage device comprising a recording head and a recording media comprising a layer of magnetic material having an anisotropy that is substantially perpendicular to a plane of the layer. The device further comprises an encoder adapted to provide an encoded data signal for recording data in encoded bit cells on the magnetic media, the encoded data signal comprising a sequence of signal transitions comprising added equalization signal transitions, the equalization signal transitions being added so that a plurality of the encoded bit cells comprises an average demagnetization field strength that is substantially null.
In another version, the data storage device comprises a recording head; a recording media comprising a layer of magnetic material having an anisotropy that is substantially perpendicular to a plane of the layer; and an encoder adapted to provide a data signal to the recording head for recording data on the magnetic media, the data signal comprising 0s represented by a first set of signal transitions and 1s represented by a second set of signal transitions, the first set being different from the second set.
In yet another version, the data storage device comprises a recording head; a recording media comprising a layer of magnetic material having an anisotropy that is substantially perpendicular to a plane of the layer; and an encoder adapted to provide an encoded data signal for recording data in encoded bit cells on the magnetic media, the encoded data signal comprising a sequence of signal transitions comprising added equalization signal transitions, the equalization signal transitions being added so that the demagnetization field strength in Oersted is less than about 75% of the saturation magnetization in Gauss.
In another aspect, the present invention comprises a method of recording data on magnetic media, the method comprising the step of selecting a magnetic media comprising a layer of magnetic material having an anisotropy that is substantially perpendicular to a plane of the layer and providing an encoded data signal for recording data in encoded bit cells on the magnetic media, the encoded data signal comprising a sequence of signal transitions comprising added equalization signal transitions, the equalization signal transitions being added so that a plurality of the encoded bit cells comprise an average magnetization field strength that is substantially null.
In another version, the method comprises the steps of selecting magnetic media comprising a layer of magnetic material having an anisotropy that is substantially perpendicular to a plane of the layer; and providing a data signal for recording data on the magnetic media, the data signal comprising 0s represented by a first set of signal transitions and 1s represented by a second set of signal transitions, the first set being different from the second set. Preferably, at least one of the first and second sets of signal transitions comprise alternating signal transitions.